Animated objects not consistently making contact with rigid body physics objects
In the 6 weeks since I posted this question, I have found solutions and/or work-arounds for the problem.

Here are the relevant points that contributed to the fix:

1) The objects that sit on the conveyor belt were actually imported from an OBJ file that I built outside of Blender.  Independent of my physics issues, I found that these objects were not rendering correctly. The issues seem to be related to normals being strangely wrong. Other users have also noted that OBJ imported objects have normals issues in Blender. I did a bulk swap-out of these imported objects using Ctrl-L Object Data, with a Blender-modeled object. This fixed my render issues.  Was this OBJ issue impacting physics also?  Perhaps.

2) The objects on the conveyor belt each have a Curve Modifier, and animation is done by key-framing the location of one of the objects that is the parent of the other objects on the conveyor belt.  The fix involved creating a set of non-rendered objects that move on a plane in concert with the objects on the belt. These super-simplified "ghost" objects are setup with rigid body physics, and their only purpose is to trigger the mallets.  I was fortunate that all of the mallets are on the same plane.  I removed physics from the belt-mounted objects.  This did the trick!!  My observation is that the critical difference is that not using a curve modifier on objects that need to be part of the physics was the break-through that I needed.  

3) Other lessons learned (painfully):   

a) I accidentally changed the scene physics speed from 1.0 to 0.1.  This caused everything to react very slowly which took me a while to find the root cause, as I was in the midst of tweaking various rigid body parameters and constraints.

b) Baking - Once I did a full bake on all frames (it took two hours), the scene simulations were much smoother.